“Hmm, that Unreal engine is pretty good!”
“But… We’ve already built a whole bunch of levels and models. Set up lighting, designed encounters, written massive amounts of new code…”
“Bah! Naysayer! Nice engine, we’ll take it!”
“But levels are constructed in an entirely different manner on Unreal! There is no way we can just port our levels!”
“You’re fired.”

The unreal engine is continually updated, but then at some point the dear Brussard (CEO of 3D Realms) finds out that id Software is working on a new engine:

“Uh, per-pixel-lighting!”
“Sir, I don’t want to poke a pike in your pie. But we’re 6 years intos development now… Our competitors have all released 3 games each in the interim, and we’re only just now about done with rebuilding out levels for the Unreal engine, remodelling our models to current standards. And I don’t even want to tell you how many times the coders have rewritten their code, I think they may be wearing down a bit on this whole thing…”
“Bah Nigel, you were always a pussy! CHANGE ENGINE!”
“Sir, the leads are all wearing t-shirts that say ‘Time to chew bubblegum and burn out on the endless self-perpetuating game development cycle, and I’m all out of gum!’. And while I know that the plan was to lengthen development so much that a new Evil Dead movie could be released and we could once again rip it off, that just doesn’t seem realistic anymore! We’ve got to ship something!’
“Doom 3 it is then.”
“But the Doom engine isn’t even capable of handling the outdoor scenes that we have been rebuilding several times already!“
“F”
“Not to mention that we’ll have to rewrite all the code again!“
“I”
“We’re the ridicule of the games industry!“
“R”
“Will you be switching to the Half-Life 2 engine once that’s released? I mean why not? We’re already rebuilding everything for the third time, so what’s a third?!“
“E”
“I quit!“

Update Again: So it turns out that Gamespot was a bit hasty, and that they aren’t using the Doom engine, but rather their own in-house engine. Changing very little of the above conversation :)